1. Field of the Invention
The present invention relates generally to batteries and, more particularly, to determining the capacity of a battery.
2. Related Art
Sudden cardiac arrest, i.e., a heart attack, has been attributed to over 350,000 deaths each year in the United States, making it one of the country's leading medical emergencies. World-wide, sudden cardiac arrest has been attributed to a much larger number of deaths each year. One of the most common, and life threatening, consequences of a heart attack is the development a cardiac arrhythmia commonly referred to as ventricular fibrillation. When in ventricular fibrillation the heart muscle is unable to pump a sufficient volume of blood to the body and, more importantly, to the brain. Ventricular fibrillation is generally identifiable by the victim's immediate loss of pulse, loss of consciousness and a cessation of breathing. The lack of blood and oxygen to the brain may result in brain damage, paralysis or death to the victim.
The probability of surviving a heart attack or other serious heart arrhythmia depends on the speed with which effective medical treatment is provided. There are four critical components of effective medical treatment that must be administered to a victim of sudden cardiac arrest: (1) early cardiopulmonary resuscitation to keep the blood oxygenated and flowing to the victim's brain and other vital organs; (2) early access to emergency care; (3) early cardiac defibrillation to restore the heart's regular rhythm; and (4) early access to advanced medical care. If prompt cardiopulmonary resuscitation is followed by defibrillation within approximately four minutes of the onset of symptoms, the victim's chances of surviving sudden cardiac arrest can approach or exceed forty percent. Prompt administration of defibrillation within the first critical minutes is considered one of the most important components of emergency medical treatment for preventing death from sudden cardiac arrest.
Cardiac defibrillation is an electric shock that is used to arrest the chaotic cardiac contractions that occur during ventricular fibrillation and to restore a normal cardiac rhythm. To administer this electrical shock to the heart, defibrillator pads are placed on the victim's chest, and an electrical impulse of the proper size and shape is administered to the victim in the form of an electric shock.
Recently, portable external defibrillators for use by first responders have been developed. A portable defibrillator allows proper medical care to be given to a victim earlier than preceding defibrillators, increasing the likelihood of survival. Such portable defibrillators may be brought to or stored in an accessible location at a business, home, aircraft or the like, ready for use by first responders. With recent advances in technology, even a minimally trained individual can operate conventional portable defibrillators to aid a heart attack victim in the critical first few minutes subsequent to onset of sudden cardiac arrest.
Portable defibrillators require a portable energy source to operate in the anticipated mobile environment. Although several manufacturers have provided customized battery packs for their portable defibrillators, some are designed to use a standard, commonly available, rechargeable battery pack, such as those used in video camcorders. These defibrillators incorporate battery interface adapters for mechanically and electrically connecting the standard battery packs to the defibrillator. The use of standard battery packs allows for a less expensive battery solution. Such battery packs may be, for example, a sealed lead acid (SLA) battery, a nickel cadmium (NiCd) battery, a lithium (Li) battery or the like.
Portable defibrillators may remain unused in their storage area for hours, days, or even weeks without having been tested or maintained by a user, and months or even years without being actually used. During such extended periods of time the battery may discharge significantly and have an insufficient charge to recharge the defibrillator during use. As a result, to ensure reliable defibrillator operations, it is critical that the condition of the battery pack be determined prior to operation. The user may then determine whether the installed or a replacement battery should be used.
Currently, three procedures or tests are commonly utilized to determine the present capacity of a battery, referred to herein as "remaining battery capacity." One conventional battery capacity test measures the time to completely discharge a fully charged battery into a known load. One drawback is that this technique requires a known load to be provided over a very long period of time. Inclusion of such a long load discharge cycle significantly increases the cost and complexity of the procedure. Another deficiency is that this test often requires hours to perform since the battery must be fully charged, and then fully discharged. Furthermore, since the battery must be completely discharged under well-known and controlled conditions, the battery powered device is unavailable for use during this test. In addition, for the battery to be used, it must be fully recharged upon completion of the test, further extending the unavailability of the battery. An additional problem is that this test decreases battery life for certain types of batteries such as NiCd or SLA batteries, reducing the number of remaining available charge cycles. Thus, although relatively accurate, this test is time consuming, inconvenient, and adversely affects the battery life and availability.
Another conventional battery capacity test, commonly referred to as an open circuit voltage test, measures the battery voltage without an attached load. This test is utilized only for certain types of batteries, such as SLA batteries, which are characterized by a predictable decrease in terminal voltage as the battery is used. The remaining battery capacity is estimated based on this decrease in voltage. However, many other battery types such as lithium batteries, silver batteries, and mercury batteries do not exhibit such a continual and predicable decrease in voltage during use. As a result, this test is not suitable for such batteries. In addition, this test is also temperature dependent. The ambient temperature will effect the battery voltage and thus the estimation of battery capacity. Also, if the battery is partially depleted, this test will be less accurate because the relationship between voltage an capacity will have changed. Thus, although relatively fast and roughly accurate, this test can only be utilized to test a minority of battery types.
A third conventional technique for testing battery capacity, commonly referred to as a battery fuel gauge, is a battery monitoring circuit that measures the current output from the battery during use, and the current input to the battery during charging cycles. The battery monitoring circuit determines the remaining capacity of the battery based on a tally of the cumulative input and output currents. This test is accurate, however, only when proper maintenance has been performed on the battery. For example, NiCd batteries require reconditioning cycles to be performed periodically. These reconditioning cycles return the all the NiCd cells within the battery pack to a full charge. Without the proper reconditioning cycles, the NiCd cells may develop a charge memory or a cell imbalance. A cell imbalance occurs when one cell of a battery pack discharges more quickly than the other cells. As the other cells in the battery pack supply current to the load, the discharged cell will be reverse charged. This reverse charge will decrease the NiCd cell life and, therefore, reduce the life of the battery pack. If the NiCd cells within the battery pack develop a charge memory, the NiCd cells will appear to be at full capacity, but in fact will be charged to only a fraction of their total capacity. Thus, the accuracy of this test is dependent upon the ability to consider the maintenance history of the battery between reconditioning procedures.
What is needed, therefore, is a fast and accurate method and apparatus for determining the capacity of a variety of battery types and chemistries under a variety of conditions.